Phonon anomalies and local atomic displacements in the exchange-enhanced superconductorU6Fe

Mössbauer effect and neutron-diffraction measurements have been made as a function of temperature on the exchange-enhanced superconductor ${\mathrm{U}}_6$Fe. Non-Debye behavior is observed in the thermal mean-squared displacement of Fe with the Debye temperature changing from ${\Theta }_f$≃320 K above 100 K to ${\Theta }_f$≃400 K below 80 K. Changes in the Mössbauer shift below 100 K are attributed principally to electronic-structure changes of the Fe atom which lead to a small increase in the number of d electrons and a concomitant reduction in the number of s electrons. The neutron-diffraction results indicate a pronounced anisotropy in the unit-cell thermal expansion and below 100 K an anomalous temperature dependence for the c-axis component of the total Debye-Waller factor of Fe. These observations taken together imply that along the c axis, static displacements of the Fe atoms begin around 100 K and increase with decreasing temperature.